Laser beam machining is a method of vaporizing or melting a workpiece by irradiating onto very small portions of the same a high-density-energy laser beam which is obtained by generating an extremely coherent, monochromatic light and then converging the light by a condenser lens. The laser beam is excellent in controllability, which enables the laser beam machining to be employed in cutting complicated shapes, very small articles with high accuracy, and so forth, under the control of a numerical control unit.
A laser beam machine for carrying out such laser beam machining suffers from a significant problem to be solved, i.e. a problem of dealing with light reflected from a workpiece. This problem is very serious especially in cutting or welding highly reflecting materials, such as aluminum and copper. Another problem is marks made by the reflected light when the machining head is moved with the shutter open for high-speed machining.
A low-loss optical resonator is formed between the workpiece and the rear mirror within a laser oscillator, which instantly amplifies the laser power of the light reflected back from the workpiece into the laser oscillator, to an extremely high level. Consequently, various kinds of mirrors arranged within the laser oscillator are damaged, which degrades the reliability of the laser beam machine.
There are two conventional methods of preventing such damage of reflecting mirrors caused by reflected light.
A first method controls input electric power such that the laser power in the laser oscillator is kept at a constant level through a fast power feedback.
A second method detects the reflected light outside the laser oscillator and interrupts the laser oscillation when the reflected light is detected in a large quantity.
Whichever of the above methods may be employed, it is required to interrupt machining when the reflected light arises. In the first method, while the total laser power inside the laser oscillator is kept at a constant level, the power of the laser beam generated by the oscillator itself is reduced by the quantity of the reflected light. A laser beam suitably used for machining is one generated by the oscillator itself. Therefore, a decrease in the power of this laser beam leads to changes in machining conditions. To avoid the changes, it is required to interrupt machining. The second method, which stops laser oscillation, necessarily interrupts machining as well.
Furthermore, both the above methods require a very expensive laser detector. And even if such a high-priced laser detector is used, it is impossible to prevent marks from being made by faint reflected light.